staging/xgifb: Consolidate XGI_EnableChISLCD and XGI_DisableChISLCD

These two functions share the same code except one line - thus we can
simply merge them and add a parameter to switch between both variants.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>